1. Field of the Invention
Development of high-performance carbon materials having light weight, high strength and high elastic modulus has been desired eagerly at present in various industrial fields such as the car industry, the airplane industry and other wide range of technical fields. Carbon fibers or molding carbon materials have been attracting an increasing attention as the materials that satisfy these needs.
The present invention relates to an optically anisotropic carbonaceous pitch suitable for the production of carbon materials containing carbonaceous fibers having light weight, high strength and high elastic modulus and other carbon materials, to a process for producing the optically anisotropic carbonaceous pitch and to a process for producing carbon fibers and graphite fibers by melt-spinning, carbonizing and graphitizing the optically anisotropic carbonaceous pitch.
2. Description of the Prior Art
Optically anisotropic pitches that have been known conventionally such as those disclosed in Japanese Patent Laid-Open Nos. 19127/1974 and 80635/1975 contain an optically anisotropic phase portion (which will hereinafter be referred to as "AP", whenever necessary) that substantially corresponds to the quinoline-insoluble content (or pyridine-insoluble content). If an AP portion content of these pitches approaches 100%, the softening point rises significantly, bringing about drawbacks that the spinning temperature becomes as high as 400.degree. C. or more and evolution of decomposition gases and polymerization of the pitches occur during spinning. In accordance with the conventional method of spinning carbon fibers, the content of the AP portion is restricted to 90% or below, especially in the range of from 50% to 70% and spinning is effected at a temperature at which thermal decomposition and thermal polymerization do not occur remarkably.
Such a conventional pitch composition is a mixture of the AP and considerable quantities of an optically isotropic phase (which will hereinafter be referred to as "IP", whenever necessary), and so is a so-called "heterogeneous pitch". Hence, yarn breaking is likely to occur during spinning, the fiber fineness becomes non-uniform and the fiber strength decreases.
The pitch material disclosed in Japanese Patent Publication No. 8634/1974 seems to consist substantially of 100% AP but is a specific pitch whose chemical structure is specified. In other words, it is a pitch which is produced by the thermal polymerization of expensive pure materials such as chrysene, phenanthrene, tetrabenzophenazine, and the like, and has a relatively regular structural molecular weight. If the pitch is produced by use of general mixture of heavy oils or tars as starting materials, the softening point of the resulting pitch inevitably becomes extremely high.
On the other hand, the pitch as the starting material of carbon fibers, disclosed in Japanese Patent Publication No. 7533/1978, has a low softening point and a low spinning temperature and spinning can be made easily. However, the reference does not disclose the AP content. In addition, the starting hydrocarbons are polycondensed by use of a Lewis acid catalyst such as aluminum chloride and hence, the composition and structure of the pitch are specific and the strength and elastic modulus of carbon fibers produced from the pitch are relatively low. The invention involves also a drawback that the catalyst used for the reaction can not be completely removed easily.
The pitch material disclosed in Japanese Patent Laid-Open No. 55625/1979 is a homogeneous pitch consisting completely of 100% AP and its molecular weight distribution is relatively narrow. As will be described below in further detail, the pitch of this type is likely to have a small content of an n-heptane-soluble component (hereinafter referred to as the "component O") and a relatively small content of an n-heptane-insoluble but benzene-soluble component (hereinafter referred to as the "component A") that are important components of the optically anisotropic pitch of the present invention. In the remaining benzene-insoluble components, the contents of a quinoline-soluble component (hereinafter referred to as the "component B") and a quinoline-insoluble component (hereinafter referred to as the "component C") are relatively large. Even if their molecular weight distribution is narrow, the softening point of the pitch material is likely at least about 330.degree. C. as its practical character, and the spinning temperature must be elevated close to 400.degree. C. Spinning on the industrial scale at such a temperature is still difficult to practice.
The pitch materials disclosed in Japanese Patent Laid-Open Nos. 160427/1979, 58287/1980, 130809/1980, 144087/1980 and 57881/1981 are obtained by solvent-extracting an isotropic pitch or a pitch containing a trace amount of AP, extracting a portion whose major component is ready to form the AP and which has a small content of the component C, and melting said portion. A pitch having uniquely the content of the component C of 25% or below can be produced. As those skilled in the art can easily assume from the process and data disclosed in the reference, however, the pitch has a high softening point so that the spinning temperature is as high as around 400.degree. C. and stable spinning on the industrial scale is still difficult. Any way, it is assumed from the process and data disclosed in the reference that the resulting pitch is a particular pitch consisting of component B as the principal component, though the content of the component C is small.
As described above, all the heretofore known, homogeneous, optically anisotropic pitches having the AP content of close to 100% have so high softening point as 330.degree. C. and can not be spun easily in a stable manner. On the other hand, the heretofore known pitches having a low softening point are heterogeneous except those which are produced from specific starting materials and have specific composition and structure. Spinning of these pitches are likewise difficult so that it has been extremely difficult to obtain carbon fibers having excellent crystaline characteristics.
In conjunction with stipulation of the pitches the conventional methods stipulate optically anisotropic pitches by means of the partial chemical structure or the average molecular weight or the quinoline-insoluble component (or pyridine-insoluble component) of the pitches. However, these simple stipulation methods are not enough, because an optically anisotropic pitch composition having a low softening point and homogeneity for the production of high-performance carbon fibers or other carbon materials can not be stipulated by these and are improper. For, so-called "optically anisotropic pitch" compositions are mixtures of compounds of numerous kinds having extremely wide range of chemical structures and a variety of molecular weight ranging from several hundreds up to several tens of thousands and even those having molecular weights comparable to those of a coke. Accordingly, the pitches can not be stipuled only by the characteristics of their partial chemical structure or the average chemical structure of the pitches as a whole.